World’s Soil Resources

9.3.6 | Soil acidification
In SSA, extremely acid soils, mainly potential or actual acid sulphate soils, occur only in a small area around
the Niger delta and sporadically along the coastal plains of West Africa. Other acid soils occupy about 15
percent of the continent and are mainly found in the moist parts of the semi-arid zones and in sub-humid
areas. Many of the Acrisols (Ultisols) and some Lixisols (Alfisols) have acid surface and subsurface horizons
which, coupled with the moisture stress conditions, makes these soils extremely difficult to manage under
low-input conditions. In West Africa, the annual additions of dust from the Sahara brought by the Harmattan
winds raise the pH of the surface horizons. The problem is therefore less acute there, although subsoil acidity
remains (Eswaran et al., 1996). Another region of acid soils occurs south of the tropic of Capricorn and includes
parts of South Africa (Beukes, Stronkhorst and Jezile, 2008a,b) where it poses a serious soil chemical problem
and is in fact one of the greatest production-limiting factors.
9.3.7 | Salinization and sodification
Salinization is defined as a change in the salinity status of the soil. This can be caused by improper
management of irrigation schemes, particularly in the arid and semi-arid regions. Irrigation-induced soil acidity
is aggravated when irrigation is practiced on soils unsuitable for irrigation (Barnard et al., 2002). Salinization
can also be caused if sea water intrudes into coastal regions either on the surface or into groundwater. It may
also arise in closed basins when there is excessive abstraction of groundwater from aquifers of different salt
content. Salinization also takes place where human activities lead to increased evapotranspiration from soils
on salt-containing parent material or where saline ground water is being pumped out (Oldeman, 2002).
In the arid and semi-arid parts of Africa, soil salinity and alkalinity are major problems affecting about 24
percent of the continent. Soils with pH>8.5 are designated as alkaline (Eswaran et al., 1996). Soil salinity and
sodicity problems are common in arid and semi-arid regions where rainfall is insufficient to leach salts and
excess sodium ions out of the rhizosphere. More than 80 million ha of such soils are found in Africa.
Increasing temperatures may result in high evaporative demands that may activate the capillary rise of
salts, leading to soil salinization. The results of a study in Sudan showed a significant increase in salinity in
the Dongla area in the north, where the annual rainfall is the lowest in the country. This increase is associated
with fluctuation and erratic distribution of rainfall, as well as with a rise in temperature (Abdalla et al., 2011).
9.3.8 | Waterlogging
Human intervention in natural drainage systems may lead to waterlogging or flooding by river water. Most
waterlogging threats are due to effects of human-induced hydromorphy. Causes include a rising water table
(for example, due to construction of reservoirs or irrigation) or increased flooding caused by higher peak flows
of rivers. The technology of flooding in paddy fields to provide a proper environment for paddy rice is generally
not considered a threat to ecosystem services, although it may increase the emissions of GHG. It is estimated
(Oldeman, Hakkeling and Sombroek, 1991) that waterlogging constitutes 1.5 percent of the non-erosion soil
degradation threats in Africa.
9.3.9 | Compaction, crusting and sealing
The population of the Sub-Saharan Africa (830 millions) is approximately 12 percent of the world population.
SSA population has been growing at a rate of 2.6 percent year during the last decade, although the rate is now
declining. The tendency in the region is towards the concentration of growing populations in moderately large
cities (rather than mega-cities). Since the early 1970s, several SSA countries have experienced accelerated urban
expansion, recording some of the highest urban growth rates in the world of up to 5 percent per year (Todaro,
2000). There are numerous examples of single-city dominance in the region. For instance, in Mozambique,
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in Africa South of the Sahara